Wind turbines are used to produce electrical energy using a renewable resource and without combusting a fossil fuel. Generally, a wind turbine converts kinetic wind energy into mechanical energy and then subsequently converts the mechanical energy into electrical energy. Wind turbines are often placed together in large groups, effectively forming wind power plants.
One common type of wind turbine is the upwind horizontal-axis wind turbine (HAWT). A HAWT includes a tower, a nacelle located at the apex of the tower, and a single turbine rotor that is supported by the nacelle. The turbine rotor is at the front of the nacelle and faces into the wind upstream of its supporting tower. The turbine rotor is coupled either directly or indirectly with a generator, which is housed inside the nacelle. The turbine rotor includes a central hub and a plurality of blades (e.g., three blades) mounted thereto and that extend radially therefrom.
The power output from any wind turbine depends on the force of the wind at the wind turbine. Generally, wind direction and velocity correlate strongly with the altitude above the earth. Higher altitudes typically equate to higher wind velocity. For example, doubling the altitude may increase wind speed by 20% to 60%. From another perspective, doubling the altitude may increase power output from a wind turbine by 34%. Thus, wind turbines that operate at higher altitudes produce comparatively more power. Yet, positioning wind turbines at higher altitudes is expensive.
To reach higher altitudes, the tower height is increased. Increasing the tower height requires an increase in the diameter of the tower to avoid buckling of the tower from the expected increase in maximum wind loading. For example, doubling the tower height may necessitate doubling the tower diameter. Doubling the tower height thus increases the amount of material by a factor of at least four. The cost of a taller tower may then be a limiting factor to elevating the wind turbine to a more optimum altitude for energy generation. Thus, there is a tradeoff between all construction costs, including the cost of the tower, and the projected power output. For HAWTs, tower heights approximately two to three times the blade length have been found to balance material costs of the tower and other components against power output.
Wind turbine design also plays a significant role in the power output from the wind at any particular altitude. In addition to the greater power from higher wind velocities, power obtained from the wind is proportional to the sweep area of the wind turbine blades. For HAWTs, sweep area is increased by using long wind turbine blades. The longer the blades, the larger the area that is traced by the blade tips. There are other wind turbine design changes that increase the sweep area.
As an alternative to HAWTs, which include a single turbine rotor, multi-rotor wind turbines incorporate multiple turbine rotors on a single support tower. Multiplying the number of rotors can effectively increase the sweep area. This amounts to simply multiplying the sweep area of one set of blades times the number of rotors (assuming all of the blades are the same length).
There are generally two types of multi-rotor wind turbines. One type is a coplanar multi-rotor wind turbine, and the other is a coaxial wind turbine. In a coplanar multi-rotor wind turbine, multiple turbine rotors are arranged in parallel with the individual wind turbine blades on each turbine rotor rotating in the same plane. This type of wind turbine may also be referred to as an array wind turbine. In a coaxial multi-rotor wind turbine, the turbine rotors are arranged in series on a single axis. That is, the wind drives a leading turbine rotor and then operates a trailing wind rotor.
In view of the multiplication of the sweep area possible with multi-rotor wind turbines, multi-rotor wind turbines offer at least the prospect of achieving much higher capacities per tower compared to single rotor wind turbines. However, multi-rotor wind turbines may also experience similar drawbacks as the HAWTs with regard to tower height. Moreover, the costs of a multi-rotor wind turbine tower may be greater still when the more complex tower system, including the weight and additional supporting equipment of multiple rotors, is taken into consideration.
Accordingly, there is a need for improved wind power plants and wind turbine systems that permit multiple wind turbines to be placed at greater altitudes while minimizing capital costs and without sacrificing the structural stability.